As is known, there are today available numerous electronic devices made at least in part of silicon carbide (SiC).
For example, there are today available metal-oxide semiconductor field-effect transistors (MOSFETs) made at least in part of silicon carbide, which is characterized by a bandgap that is wider than the bandgap of silicon. Consequently, given the same doping level, the critical electrical field of silicon carbide is higher than the critical electrical field of silicon. For example, in the case where silicon carbide has a so-called 3C polytype, the critical electrical field is approximately equal to four times the critical electrical field of silicon; in the cases where silicon carbide has instead a 6H or a 4H polytype, the critical electrical field is, respectively, approximately eight times and ten times the critical electrical field of silicon.
Thanks to its high critical electrical field, silicon carbide enables provision of junctions having breakdown voltages higher than what may be obtained using silicon. Furthermore, exploiting the high critical electrical field, it may be possible to provide transistors with drift regions having thicknesses smaller than the drift regions of traditional silicon transistors; said transistors are hence characterized by low on-resistances (Ron).
On the other hand, silicon carbide has a low diffusiveness of the dopant species, even at high temperatures. In addition, as compared to silicon, silicon carbide is characterized by a reduced mobility μ of the carriers. In fact, in silicon carbide, the mobility μ of the carriers is typically of the order of some hundreds of cm2/Vs, whereas, in silicon, the mobility μ of the carriers can exceed even thousands of cm2/Vs. In particular, in the case of MOSFETs made of 4H-polytype silicon carbide, the mobility μ of the carriers in the respective channel regions is limited to approximately 50 cm2/Vs, on account of the generation of states at the oxide-semiconductor interfaces.
In greater detail, there are today available electronic devices formed starting from a silicon-carbide substrate. However, the technology today available does not enable provision of silicon-carbide wafers with diameters larger than four inches; consequently, the manufacture of said electronic devices is generally more costly and technologically complex than the manufacture of electronic devices starting from silicon substrates.
In order to combine the advantages of silicon and silicon carbide, electronic devices have moreover been proposed formed starting from a silicon substrate and comprising one or more silicon-carbide epitaxial layers. For example, U.S. Pat. No. 5,877,515, which is incorporated by reference, describes a semiconductor device, and in particular a MOSFET, having a silicon layer, which is deposited on a silicon-carbide layer, which in turn is deposited on a silicon substrate.
Operatively, the silicon-carbide layer enables a concentration of charge to be obtained that is higher than what may be obtained in the case of a silicon layer, given the same breakdown voltage. However, it may be possible that in certain conditions, and in particular in the case where the semiconductor device is biased so as to work in the region of inhibition, a non-negligible electrical field is generated within the silicon substrate. In said conditions, it is the silicon itself that limits, with its own critical electrical field, the breakdown voltage of the semiconductor device.
In order to prevent generation of a non-negligible electrical field within the silicon substrate, it may possible to increase the thickness of the silicon-carbide layer; however, said operation, in addition to being technologically complex, entails an increase of the on-resistance of the semiconductor device.